1. Field of the Invention
This invention relates to a counter flow device for an electroplating apparatus for metallic strips, and more particularly to a counter flow device for an electroplating apparatus having a radial cell type plating bath or tank capable of high current density plating the metallic strips running through the bath at low speeds.
2. Description of the Prior Art
A radial cell type plating apparatus includes a large diameter rotary drum for current flow with its substantially half part immersed in a plating solution or electrolyte. A metal strip is brought into contact with a substantially half circumference of the drum and passes thereabout in synchronism with the rotation of the drum during which electric current is caused to flow through the plating solution between the strip and an anode spaced apart therefrom by a radial current flow gap.
Such a plating apparatus is advantageously used to plate only one surface of a strip owing to its inherent construction and permits a distance between the strip and an anode to be possibly small so as to avoid superfluous consumption of the plating electric power, thereby enabling high speed plating with high power to be effected.
In such an electroplating system, there are generally two cases, one using insoluble electrodes as anodes, and the other using soluble electrodes consisting mainly of a metal the same as a plating metal. Particularly, the latter case using the soluble electrodes has advantages in that the plating metal is easily replenished and gas evolved from the electrode surfaces is little, so that it is suitable for obtaining thick plating layers with high power.
In this plating system hitherto used, as schematically shown in FIGS. 1a and 1b, a plating solution or electrolyte is jetted upward from an inlet 2' at the bottom of a plating tank 2 toward a rotary drum 3 against a strip 1 so as to be supplied into clearances between a metallic strip 1 and a pair of anodes 5. The metallic strip 1 is in contact with an outer circumference of the rotary drum 3 and carried along with the rotating drum 3. The pair of anodes 5 are arcuate in section and arranged side by side in a moving direction of the strip and in opposition to a lower half circumference of the drum 3.
Accordingly, the plating solution flows against the movement of the strip on the entrance side of the strip (referred to hereinafter "downpass") but flows in the same direction as the movement of the strip on the exit side of the strip (referred to hereinafter "uppass").
It has been regarded that high current density in electroplating is preferable because required plating is obtained at a higher plating speed or in a smaller plating apparatus. When the current density is too high beyond a limit value, however, treelike electric deposits often occur on surfaces of metallic strips and in particularly, defects are frequently caused in edges of the strips called "scorching" or "black edge" due to the excess concentration of electric current.
Such a critical current density varies with plating conditions such as compositions and temperatures of the plating solution, among which relative speeds between the strips and the plating solutions greatly affect the critical current density.
In the radial cell type plating bath or tank of the prior art as shown in FIG. 1, therefore, the relative speed between the metallic strip and the plating solution becomes much smaller in the above mentioned "uppass" where the strip and plating solution move in the same direction, particularly with a low moving speed of the strip, to cause the critical current density to be much smaller, whereby there is a great tendency to cause the "scorching" in the edges of the strips. As shown in FIG. 2 illustrating one example of the relation between the critical current density and the speed of metallic strip, even under conditions good for plating in the "downpass", the "scorching" occurred in the "uppass". It was therefore required to decrease all the supply current and to lower the speed of metallic strip in order to avoid the "scorching". In other words, it could not help doing a disadvantageous operation for plating the metallic strip.
Recently, moreover, the requirement for the corrosion resistance has become severer and various kinds of alloy plating instead of hitherto used single metal plating have been put into practical use. It has been studied for plating to use for example not only binary alloys such as Zn-Ni, Zn-Fe and the like but also multiple metal alloys such as Zn-Ni-Co, Zn-Ni-Cr and the like.
In plating using these alloys, deposits of components of the alloys are delicately affected by plating conditions. In other words, the variation in current density and flowing speed of electrolyte greatly affects compositions of the plating alloys. Referring to FIG. 3a illustrating one example of Zn and Fe distributions in Zn-Fe alloy plating layer by IMMA plated by the hitherto used radial cell type plating apparatus, it is recognized that the contents of Fe and Zn considerably varies in a direction of thickness or depth. In this case, it has been experienced that sometimes unstable black stripe patterns occur on the surfaces of the strips probably caused by irregular flow velocity of the electrolyte, which considerably spoil the appearance of the metallic strips.